The purpose of this study is to observe the effect of giving three and four study medications together on the amount of HIV virus in a child's blood. The combination of three medicines is zidovudine (ZDV), lamivudine (3TC), and nevirapine (NVP). The combination of four medicines is ZDV, 3TC, NVP, and 1592U89. The goal is to see if subjects will have an undetectable amount of HIV virus in the blood by week 12. Another purpose is to identify how long after week 12 the amount of virus in the blood remains undetectable. This study will also determine the safety of three- and four-investigational drug combinations.